The objective of the proposed experiments is to describe as fully as possible changes which occur in the class of abundant messenger RNA during development in the sea urchin. Using molecular hybridization techniques we will examine the persistence of abundant maternal messenger RNA species and the time of appearance of new abundant mRNAs during embryogenesis. After analysis of the general patterns of sequence flow into and out of the abundant mRNA class, we will isolate DNA sequences representing individual mRNA species as recombinant DNA clones. These clones will furnish sensitive probes for determining the time of appearance of and monitoring changes in the concentration of single mRNA species during development. We will apply in situ hybridization methods to examine the localization of cloned sequences in specific cell types. We will follow changes in localization of maternal and newdy synthesized abundant mRNA species during the course of development. We hope to identify structural gene sets whose transcription is coordinated and whose products are localized to specific cell types or their progenitors during early development.